Refrigerant vapor compression systems are well known in the art and commonly used for conditioning air to be supplied to a climate controlled comfort zone within a residence, office building, hospital, school, restaurant or other facility. Refrigerant vapor compression systems are also commonly used in transport refrigeration systems for refrigerating air supplied to a temperature controlled cargo space of a truck, trailer, container or the like for transporting perishable items. Traditionally, most of these refrigerant vapor compression systems operate at subcritical refrigerant pressures and typically include a compressor, a condenser, and an evaporator, and expansion device, commonly an expansion valve, disposed upstream, with respect to refrigerant flow, of the evaporator and downstream of the condenser. These basic refrigerant system components are interconnected by refrigerant lines in a closed refrigerant circuit, arranged in accord with known refrigerant vapor compression cycles, and operated in the subcritical pressure range for the particular refrigerant in use. Refrigerant vapor compression systems operating in the subcritical range are commonly charged with conventional fluorocarbon refrigerants such as, but not limited to, hydrochlorofluorocarbons (HCFCs), such as R22, and more commonly hydrofluorocarbons (HFCs), such as R134a, R410A and R407C.
In today's market, greater interest is being shown in “natural” refrigerants, such as carbon dioxide, for use in air conditioning and transport refrigeration systems instead of HFC refrigerants. However, because carbon dioxide has a low critical temperature, most refrigerant vapor compression systems charged with carbon dioxide as the refrigerant are designed for operation in the transcritical pressure regime. For example, transport refrigerant vapor compression systems having an air cooled refrigerant heat rejection heat exchanger operating in environments having ambient air temperatures in excess of the critical temperature point of carbon dioxide, 31.1° C. (87.8° F.), must also operate at a compressor discharge pressure in excess of the critical point pressure for carbon dioxide, 7.38 MPa (1070 psia) will operate in a transcritical cycle. In refrigerant vapor compression systems operating in a transcritical cycle, the refrigerant heat rejection heat exchanger operates as a gas cooler rather than a condenser and operates at a refrigerant temperature and pressure in excess of the refrigerant's critical point, while the evaporator operates at a refrigerant temperature and pressure in the subcritical range.
Conventional subcritical refrigerant vapor compression systems charged with conventional fluorocarbon refrigerants may also include a receiver disposed in the refrigerant circuit downstream of the condenser and upstream of the expansion device. Liquid refrigerant from the condenser enters the receiver tank and settles to the bottom of the tank. As this liquid will be at saturated temperature, refrigerant vapor will fill the space in the tank not filled by liquid refrigerant. Liquid refrigerant is metered out of the receiver tank by the expansion valve which controls refrigerant flow to the evaporator. As the operating conditions of the subcritical refrigerant vapor compression system change, the charge requirements for the system will change and the liquid level in the receiver tank will rise or fall, as appropriate, to establish a new equilibrium liquid level.
U.S. Pat. No. 5,174,123 discloses a subcritical refrigerant vapor compression system for transport refrigeration applications that includes a compressor, a condenser, and an evaporator, with a flash tank disposed between the compressor and the evaporator. Refrigerant flows into the flash tank from the condenser at saturated conditions. The flow of refrigerant into the flash tank is controlled by selectively opening or closing a sub-cooling valve to maintain a desired degree of sub-cooling. The flow of liquid refrigerant out of the flash tank to the evaporator is controlled by a suction superheat thermostatic expansion valve. Refrigerant vapor collecting in the flash tank above the liquid refrigerant therein is returned to the compressor, being injected into an intermediate pressure stage of the compressor.
In a transcritical refrigerant vapor compression system, however, controlling the system refrigerant charge is more complex because the compressor high side refrigerant leaving the gas cooler is above the refrigerant's critical point and there is no distinct liquid or vapor phase and thus the charge present in the receiver becomes a function of temperature and pressure which may not respond in a desirable manner to system charge requirements. U.S. Pat. No. 6,385,980 discloses a transcritical refrigerant vapor compression system including a flash tank disposed between a gas cooler and an evaporator and a controller regulating valves in response to the sensed refrigerant pressure in the gas cooler to control the amount of charge in the flash tank to regulate the refrigerant pressure in the gas cooler. The flow of supercritical refrigerant from the gas cooler into the flash tank is controlled by regulating an in-line expansion valve on the entry side of the flash tank and the flow of liquid refrigerant from the flash tank to the evaporator is controlled by an in-line expansion valve on the exit side of the flash tank. Refrigerant vapor collecting in the flash tank above the refrigerant liquid therein is returned to an intermediate pressure stage of the compression device. Thus, the flash tank functions both as an economizer and a refrigerant charge control device.